1. Field of the Invention
The present invention relates to an organic electro luminescence element (also referred to as an “organic EL element”).
2. Description of the Background Art
A general active matrix type organic EL element has a bottom emission structure in which light is taken out from the side of a TFT array substrate. This structure has a disadvantage that part of the light to be emitted is shielded by a pixel circuitry, that is, numerical aperture becomes smaller, which problem becomes more serious as the definition becomes higher. To address this problem, a top emission structure has been proposed, in which the light is taken out from the top surface that is opposite to the TFT array substrate, in order to improve the numerical aperture of the organic EL element.
By way of example, Japanese Patent Laying-Open No. 2001-43980 discloses an organic EL element having the top emission structure formed by placing an organic EL layer including a light emission layer, a hole transporting layer and the like, between an anode that reflects light and a cathode that transmits light. In this organic EL element, a metal belonging to group 5 or 6 of the periodic table, such as Cr, is used as a material of the anode. Here, it is indicated that work function of the anode is as low as less than 4.8 eV. When holes are to be injected to the hole transport layer having ionization potential of 5.0 eV or higher, the anode that has such a small work function causes a problem that injection efficiency decreases and that driving voltage increases.
As an improvement of the technique disclosed in Japanese Patent Laying-Open No. 2001-43980 mentioned above, Japanese Patent Laying-Open No. 2002-198182 discloses a structure in which Au, which is a material having high work function, is deposited to the thickness of 1 to 10 nm as a hole injecting thin film layer, on a light-reflecting anode portion. This is to increase the hole injection efficiency, and to have the anode portion bear the role of reflecting light, by increasing transmittance of the Au film. This structure, however, has a problem that a metal such as Au having high work function described as a thin film layer for hole injection is not suitable for fine processing such as lithography. A metal having high work function such as Au is chemically stable, and it is difficult to remove by etching. This is the main reason why such metal is not suitable for fine processing. In view of the foregoing, in Japanese Patent Laying-Open No. 2002-198182 mentioned above, a shadow mask is employed for selective vapor deposition at an opening only, when the thin film layer for hole injection is vapor-deposited, so that a pattern of a metal film having high work function is formed. Even in this manufacturing method, however, pattern-processing accuracy is low and insufficient for attaining higher definition. Further, this manufacturing method has a problem that highly accurate alignment of the shadow mask becomes necessary to manufacture the element having the above described structure, which leads to lower productivity. After all, there still remains the problem that fine processing of a metal film having high work function is difficult for forming the thin film layer for hole injection, and a solution has not been found.
As described above, in the conventional top emission structure, such an anode structure that enables higher efficiency of hole injection to the organic EL layer, having light reflection characteristic for taking out light from the organic EL layer to the front surface and allowing formation of a high definition pattern has not been found. Therefore, there has been an essential problem that even when the numerical aperture could be improved by the adoption of the top emission structure, general performance could not be enhanced.